ES2691938T3 - Thermal spraying of ceramic materials - Google Patents

Abstract

A process for thermally spraying ceramic particles coated with metallic oxide on a substrate comprising: (i) obtaining a plurality of particles coated with metallic oxide of silicon carbide, silicon nitride, boron carbide or boron nitride in which the coating metal oxide forms at least 5% by weight of the weight of the coated particle; and (ii) thermally pulverize the particles of step (I) on a substrate; in which the metal oxide is yttrium and aluminum garnet and in which the coating forms a continuous and complete coating around the ceramic particle.

Description

5

10

fifteen

twenty

25

30

35

40

Four. Five

fifty

55

60

65

DESCRIPTION

Thermal pulverization of ceramic materials

The present invention relates, according to claim 1, to a process for thermally spraying particles of silicon or boron carbide coated with metal oxide or particles of silicon or boron nitride coated with metal oxide on a substrate to provide substrates valuable coated with these ceramics. Preferred aspects of the invention also relate to a process for manufacturing these oxide-coated ceramics with sufficiently thick coatings, so that the coatings are capable of protecting the core of ceramic particles during the thermal spray operation.

Background

Materials based on nitride and silicon carbide and boron had been widely used in many industries due to their excellent combination of mechanical, thermal and chemical properties. These carbides and nitrides offer very good tribological properties and resistance to corrosion and, therefore, are generally used in coating applications that require resistance to wear and abrasion, for example, in a corrosive environment. They compare favorably with more expensive materials such as diamond in terms of these properties.

Silicon carbide, for example, is widely used as a protective coating in industrial applications such as aerospace mobile components, metalworking tools and petrochemical pipes. This has made these ceramics an attractive synthetic target for inorganic chemicals.

Most of the carbide and silicon nitride and boron coatings are generally deposited on a substrate by physical vapor deposition (PVD) or vapor deposition (CVD). These methods are expensive, time-consuming and limited to small items that fit into the deposition chamber. The methods often require complex processing conditions.

The thermal and kinetic spraying processes in general have been accepted as one of the most effective and economical methods to produce metallic and ceramic coatings in small to large scale components. However, these methods are not always suitable for depositing carbides or ceramic nitrides due to the decomposition or sublimation of the metal and carbide species at the temperatures necessary to thermally pulverize them. This is true for cases of silicon and boron carbides and their nitrides.

However, there are some processes for thermal coating with SiC in the literature. Powders of 50-60% by volume of SiC + Ni / Co can be alloyed mechanically with high energy grinding. A high-speed oxy-fuel process (HVOF) (a type of thermal spraying process) can be used to produce SiC coatings (see Wielage, J. et al., International Thermal Spray Conference 2002, E. Lugscheider, ed. : DVS-ASM International, (2002), pp. 1047-1051). However, the components of this process exist as separate phases within the mixture. In this case there is no particle coating process.

Alternatively, powders of 67% by weight of SiC + 21.2% by weight of Al2O3 and 11.8% by weight of Y2O3 can be mixed, agglomerated and sintered. This mixture can be thermally sprayed using a detonation gun, an atmospheric plasma spray or a high-speed oxy-fuel process (HVOF) to produce SiC coatings (see WO 03/004718). The process involves mixing these materials by grinding and drying by spraying to produce agglomerated particles and then sintering in an inert atmosphere. The formed particles can then be thermally sprayed. Keep in mind that metal oxides and the ceramic component form separate phases in this process. There is no possible coating in this process.

A more limiting process involves a mixture of about 60% by weight of SiC + 40% by weight of boride binder selected from zirconium boride (ZrB2), titanium boride (TiB2) or hafnium boride (HfB2) produced by spray drying (document US20040258916). This process must be carried out in the absence of oxygen, so it is not practical at the industrial level.

The main problems that must be addressed when preparing a SiC coating are its sublimation (at around 2500 ° C) and its decomposition (also around the 2500 ° C mark). The particles are exposed to these temperatures during thermal spraying. To prepare adequate particles of SiC and other ceramics, it is also necessary to sinter the ceramics and that also poses problems. Ensuring a homogeneous distribution of the sintering adjuvant is key to achieving a good particulate product and that is difficult.

The present inventors seek to avoid the problems of decomposition and sublimation of the ceramic material by encapsulating the ceramic in a relatively coarse oxide coating.

5

10

fifteen

twenty

25

30

35

40

Four. Five

fifty

55

60

65

Possibly ceramic particles coated with metal oxide are known. US5098740 describes a process for coating ceramic particles such as SiC or silicon nitride with a coating of metal hydroxide or metal oxide. The coatings provided in US5098740, however, serve purely to provide a homogenous distribution of the sintering aids. The idea is to provide sintering additives through a coating with the lowest additive content possible. It is not believed that the coatings of US5098740 are thick enough to protect the core particle during thermal spraying.

WO 03/004718 A2 discloses a thermal spraying process of ceramic particles coated with metal oxide on a substrate comprising: I) obtaining a plurality of YAG-coated particles of silicon carbide and thermally spraying the particles on a substrate.

The present inventors have realized that oxide-coated ceramic particles are key to allowing thermal spraying. The inventors have devised a process for the formation of ceramic particles coated with metal oxide and have shown that they can be thermally sprayed without decomposition or sublimation of the nuclear ceramic particle. Therefore, the process of the invention opens the door to comparatively cheap SiC type coatings on a wide variety of substrates.

It is a further object of the present invention to provide a method for forming carbide or silicon nitride and boron coatings on a substrate using a thermal spraying apparatus.

A key aspect in the formation of ceramics coated with metal oxide is the formation of an intermediate particle not coated with oxide whose coating becomes an oxide through calcination and sintering. The present disclosure is based, inter alia, on the formation of an intermediate coating based preferably on a metal hydroxide or metal carbonate or on a mixture of metal hydroxide and metal carbonate. These species are preferably generated from another salt such as a nitrate.

Summary of the invention

According to claim 1, the invention provides a process for thermally spraying ceramic particles coated with metal oxide onto a substrate.

Considered from another aspect, a process for thermally spraying ceramic particles coated with metal oxide onto a substrate comprising:

(I) obtaining a plurality of particles coated with metal salts of silicon carbide, silicon nitride, boron carbide or boron nitride, such as particles coated with metal hydroxide and / or metal carbonate

(II) calcining and sintering the particles of step (I) to form a plurality of particles coated with metal oxide of silicon carbide, silicon nitride, boron carbide or boron nitride; Y

(III) thermally pulverizing the particles of stage (II) on a substrate.

Considered from another aspect, a process for thermally spraying ceramic particles coated with metal oxide onto a substrate comprising:

(I) obtaining a plurality of particles coated with silicon carbide metal hydroxide, silicon nitride, boron carbide or boron nitride;

(II) calcining the particles of step (I) to form a plurality of particles coated with metal oxide of silicon carbide, silicon nitride, boron carbide or boron nitride; Y

(III) thermally pulverizing the particles of stage (II) on a substrate.

Considered from another aspect, a process for thermally spraying ceramic particles coated with metal oxide onto a substrate comprising:

(I) obtaining a plurality of particles of silicon carbide, silicon nitride, boron carbide or boron nitride;

(II) combining the particles of step (I) with at least one metal salt, such as two metal salts, in the presence of a weak acid or a weak base to form a metallic salt coating on said particles;

(III) drying, such as spray drying, the particles of step (II);

(IV) calcining and sintering the particles of step (III) to form a plurality of particles coated with metal oxide of silicon carbide, silicon nitride, boron carbide or boron nitride; Y

(V) thermally spraying the particles of stage (IV) onto a substrate.

Considered from another aspect, a process for thermally spraying ceramic particles coated with metal oxide onto a substrate comprising:

5

10

fifteen

twenty

25

30

35

40

Four. Five

fifty

55

60

65

(I) obtaining a plurality of particles of silicon carbide, silicon nitride, boron carbide or boron nitride;

(II) combining the particles of step (I) with at least one metal salt in the presence of a weak base to form a coating of metal hydroxide on said particles;

(III) drying, such as spray drying, the particles of step (II);

(IV) calcining the particles of step (III) to form a plurality of particles coated with metal oxide of silicon carbide, silicon nitride, boron carbide or boron nitride; Y

(V) thermally spraying the particles of stage (IV) onto a substrate.

Considered from another aspect, a process is provided to prepare ceramic particles coated with metal oxide comprising:

(I) obtaining a plurality of particles of silicon carbide, silicon nitride, boron carbide or boron nitride;

(II) combining the particles of step (I) with at least one metal salt in the presence of a weak acid or a weak base to form a metallic salt coating on said particles;

(III) drying, such as spray drying, the particles of step (II); Y

(IV) calcining and sintering the particles of stage (III) to form a plurality of particles coated with metal oxide of silicon carbide, silicon nitride, boron carbide or boron nitride.

Considered from another aspect, a process is provided to prepare ceramic particles coated with metal oxide comprising:

(I) obtaining a plurality of particles of silicon carbide, silicon nitride, boron carbide or boron nitride;

(II) combining the particles of step (I) with at least one metal salt in the presence of a weak base to form a coating of metal hydroxide on said particles;

(III) drying, such as spray drying, the particles of step (II); Y

(IV) calcining the particles of step (III) to form a plurality of particles coated with metal oxide of silicon carbide, silicon nitride, boron carbide or boron nitride.

Considered from another aspect, an article having a coating applied thereon is provided by a thermal spraying process as defined above.

Considered from another aspect, the use of particles coated with metal oxide of silicon carbide, silicon nitride, boron carbide or boron nitride to thermally spray onto a substrate is provided.

Considered from another aspect, particles coated with metal oxide of silicon carbide, silicon nitride, boron carbide or boron nitride are provided in which the amount of metal oxide is at least 10% by weight, such as at least 20% by weight. The upper limit of the metal oxide may be 40% by weight of the total weight of the particles, such as up to 35% by weight, especially up to 30% by weight, such as those prepared by the processes described herein above. .

Ideally, a process is provided for preparing ceramic particles coated with metal oxide comprising:

(I) obtaining a plurality of particles of silicon carbide, silicon nitride, boron carbide or boron nitride;

(II) combining the particles of step (I) with at least two metal nitrates in the presence of a weak acid or a weak base;

(III) drying, such as spray drying, the particles of step (II); Y

(IV) calcining and sintering the particles of stage (III) to form a plurality of particles coated with metal oxide of silicon carbide, silicon nitride, boron carbide or boron nitride.

Definitions

The term thermal spraying is used herein to cover spraying using a thermal spraying process by combustion, a thermal spray by detonation process (such as high frequency pulse detonation), or an electric / thermal spraying process. plasma. These techniques are not new and are familiar to workers in this field.

The term weak base or weak acid is used to require the presence of a base or chemical acid that is not completely ionized in an aqueous solution. A metal salt is an ionic compound of at least one metal ion and at least one anion. That anion can be organic or inorganic, preferably inorganic.

A metal hydroxide according to the invention is a compound containing a metal ion and an OH- ion. It can also contain other anions. Thus, the boehmite compound, AIOOH, is considered a hydroxide in this document.

5

10

fifteen

twenty

25

30

35

40

Four. Five

fifty

55

60

65

A metal carbonate according to the invention is a compound that contains a metal ion and a CO32- ion. It can also contain other anions.

Detailed description of the invention

This invention relates to particles of silicon carbide (SiC), boron carbide (B4C), silicon nitride (Si3N4) and boron nitride (BN) coated with a metal oxide to allow its application on a substrate by thermal spraying without sublimation or decomposition of the ceramic core. Although the invention will generally be described herein with reference to the term ceramic particles, it will be understood as meaning SiC, silicon nitride, boron nitride or boron carbide. The use of Si in ceramics, and more especially the use of SiC, is the most preferred option.

The coating of metal oxide that we apply is thick enough to allow the thermal pulverization of the particles when protecting the core, but, naturally, the oxide coating will also act as a sintering additive during the sintering process. The metal oxide coating will provide a matrix phase and will act as a wetting agent that melts during the spraying process. The molten oxide binds the SiC particles together and on a substrate. It also provides a high cohesive resistance between the particles.

The process of the invention begins with ceramic particles such as SiC. The particle size is usually in the order of 50 nm to 5 micrometres (microns), such as 200 nm to 5 micrometres (microns) at this point, preferably 400 to 3500 nm. The particles are preferably not agglomerated at this time. The particles preferably flow freely and, therefore, are in the form of a powder or in the form of a stable suspension. These particles are well known and can be purchased in the open chemical market. However, these particles can not be thermally sprayed directly, since they decompose and sublimate at the temperatures at which the particles are exposed during the spraying process. Even sintered SiC in general can not be thermally sprayed.

The inventors have realized that there is a solution to this problem by providing a sufficiently thick oxide coating on the particles. This coating must be able to avoid the decomposition or sublimation of the ceramic particle during thermal spraying. Therefore, the coating is not only present as a homogeneous sintering aid, although it also performs this function, thus avoiding the need for more present sintering aids.

In general, it is believed that, to ensure that there is a sufficiently thick coating on the ceramic particles, there must be at least 5% by weight of the coating present, preferably at least 10% by weight, especially at least 20% by weight , especially at least especially at least 30% by weight on a coated partridge. Values in the range of 5 to 40% by weight are contemplated, such as from 7.5 to 35% by weight, especially from 10 to 30% by weight. Naturally, the thickness of the coating required may depend on the thermal spray conditions used.

In order to introduce an oxide coating on the ceramic particles, the present inventors have realized that this can be achieved by the calcination and sintering of a precursor coating. Ideally, the precursor coating is formed by the precipitation of at least one metal salt on the substrate of ceramic particles, by the precipitation of at least one metal salt on the ceramic particles or by spray drying a mixture of at least one metallic salt and ceramic particles. Ideally, there will always be two or more metallic salts, however, it is possible that a salt is used.

Therefore, in order to introduce an oxide coating into the ceramic particles, the present inventors have realized that this can be effected during the calcination of a coating without oxide. The metal salt used, therefore, preferably is not an oxide. In particular, the inventors tried to include a hydroxide or carbonate precursor coating (or ideally a mixture of hydroxide / carbonate) on the particles. When the precursor coating is calcined in the presence of oxygen, this becomes an oxide coating.

The inventors have also realized that there are several ways to provide a coating of metal hydroxide and / or carbonate or other salt-based coating on the ceramic particles. This can be achieved by co-precipitating the precursor of the metal salt on the particles or by precipitating the metal salt on the ceramic particles or by spray drying an appropriate mixture.

Therefore, it is preferred that the ceramic particles are contacted with 5 to 50% by weight, preferably 7.5 to 40% by weight, such as 10 to 35% by weight, especially 11 to 30% by weight. weight of a salt or metal salts or metal salt sol. In some embodiments, there must be more than 10% by weight of the salt or metal salts or metal salt sol present. Therefore, if there is 1 g of ceramic particles (solid in any carrier medium), 40% by weight of metal salts represents 400 mg. The metals in the salt or salts used in the invention are Al and Y. The counterion is preferably not an oxide, but is a counterion that can preferably be converted to hydroxide or carbonate (if necessary) and then into an oxide during the process of invention.

5

10

fifteen

twenty

25

30

35

40

Four. Five

fifty

55

60

65

The preferred counterions are, therefore, nitrates, halides, sulfates, sulfides and nitrites. A hydroxide or carbonate can also be used directly. The use of nitrates is especially preferred.

In order to ensure a successful coating operation, the salt is preferably one that is deposited on the ceramic particles such as a hydroxide or carbonate during the coating process or at least converted to said hydroxide or carbonate during the coating process. The presence of a hydroxide or carbonate precursor coating is the key to the subsequent formation of an oxide coating.

In a further preferred embodiment, it is preferred if a mixture of metal salts is used as an oxide coating precursor. In particular, the use of two different salts is preferred. When two salts are used, it is preferred that the metal ions be different. It is also preferable that the two counterions are equal. Therefore, the use of two different metal nitrates is especially preferred.

The metal salts of interest are preferably soluble in the solvent used during the process, especially soluble in water.

The highly preferred metal salts of use in the invention are Al (NO3) 3, (gives AhO3), Y ((NO3) 3 (from Y2O3), in particular, the salts can be hydrates, Preferred salts are Al (NO3) 3 ■ nH2O, Y (NO3) 3 ■ nH2O, AlCh ■ nH2O, YCb ■ nH2O, Y2 (CO3) 3 ■ nH2O.

Ideally, when two metal salts are present, the combination of the metal salts forms a eutectic of metal oxides after calcination. Therefore, the amount of metal salts added to the ceramic is carefully measured so that a eutectic system is formed. A eutectic system is a mixture of compounds or chemical elements that has a unique chemical composition that solidifies at a lower temperature than any other composition composed of the same ingredients. In this field, the person skilled in the art is aware of certain combinations of metal salts that form eutectics. For example, the use of certain proportions of aluminum nitrate and yttrium nitrate forms a yttrium-aluminum garnet eutectic after calcination (YAG, Y3Al5O12).

In a first embodiment, the metal oxide precursors are introduced into the ceramic particles by coprecipitation. The coprecipitation of the salt precursor or metal salts can be carried out by mixing the ceramic particles with the precipitating compound or compounds in an aqueous suspension, such as one containing from 3 to 10% by weight of solid content, preferably about 5% by weight. % in weigh. The suspension can be stirred to decompose any agglomerate and to homogenize and disperse the ceramic particles.

The mixed suspension can then be heated from 50 ° C to 100 ° C, preferably around 90 ° C to assist the precipitation process. The solution of salt or eutectic metal salts can be supplied in the mixed suspension in any order. However, an inverse titration method is preferred in which the eutectic salts are added in a controlled order. The use of a precipitating compound is preferred to ensure the activation of a hydroxide or carbonate precipitation which ideally forms a coating on silicon carbide particles during the process.

Alternatively, the metal salts, the precipitator and the particles can be combined and spray dried to introduce a coating on the particles, in particular when a weak acid is used as a precipitator. Spray drying can provide more spherical particles and, therefore, allow better fluidity.

Therefore, the key to a successful coating operation is the presence of a "precipitator" compound that allows the precipitation of the metal salts on the ceramic particles. This compound is a weak acid or a weak base. The precipitating compound may be present in a molar amount of about 1 to 30 times, preferably 3 to 30 times, such as 5 to 30 times the molar amount of salt or metal salts present, preferably 6 to 20 times, especially 5 to 10 times, such as 8 to 10 times.

When a weak acid is used, the molar ratio of precipitator to total metal cation is preferably 1 to 3. When a weak base is used, an ideal molar ratio of precipitator to total metal cation is 6 to 8.

In some embodiments, it is preferred that the amount of precipitant compound present be such that the pH of the mixture is basic, for example, pH 9-11. Ideally, during the coating process, the pH of the suspension is 9 or more when a weak base is used as a precipitator. When a weak acid is used, pH values as high as 1 to 2 can be used.

The precipitating compounds of interest are weak acids such as alkanoic acids (ethanoic acid, methanoic acid), HF, formic acid and organic acids such as dicaric acid. The use of dehydric acid is especially preferred. Alternatively, the preferred compounds are weak bases such as ammonium hydroxide, alkylamines, but in particular urea, ammonia solution and hydrogen carbonates such as ammonium hydrogencarbonate. Ideally, the precipitator compound is soluble in water. The use of urea or ammonium hydrogencarbonate is especially preferred.

5

10

fifteen

twenty

25

30

35

40

Four. Five

fifty

55

60

65

When the particles are spray dried, it is preferred that the precipitator compound be a weak acid, such as citric acid, since this gives the best final agglomerated SiC powders as compared to when a weak base is used.

In another embodiment, the metal oxide coating is produced by the precipitation of a metal salt sol such as a hydroxide sol. The ceramic particles are mixed with the metallic salt sol to form a mixed suspension, such as one containing a total solid charge of 3 to 20% by weight, such as 3 to 10% by weight of the ceramic particles, such as approximately 5% by weight or 10% by weight of ceramic particles. The solvent is preferably water. The agitation can be reused to homogenize the suspension. The precipitator compound is then added in a controlled order, preferably using titration to assist the precipitation of the metal salt sol layer onto the ceramic particles. The heating of the mixed suspension and the control of the pH are part of the process that defines the successful precipitation with metal salt sol. It is preferred to heat the suspension between 50 ° C and 100 ° C, preferably around 90 ° C to aid the process. The pH can be maintained at levels below 2 if a weak acid is used or at 9 or more, such as from 9 to 11 if a weak base is used.

Preferred metal sol precursors are inorganic metal salts or metal organic compounds such as metal alkoxides, boehmite [AlO (OH)] or basic yttrium carbonate [Y (OH) CO3].

The amount of deposition is a function of the amount of metallic salts or metal salt sol added according to the calculation of the molar ratio to the percentage by weight. More salt (es) in the system gives (n) a thicker layer.

This process can take place at room temperature. However, the temperature of activation of the precipitator is preferably 50 ° C to 100 ° C. For ammonium hydrogencarbonate, a temperature of about 50 ° C is preferred. For urea and citric acid, a preferred temperature is about 90 ° C. In addition, the pressure can be environmental.

However, it may be necessary to use a dispersant in the mixed suspension to disperse the ceramic particles and avoid agglomeration in the presence of the precipitator and during the addition of the salt or metal salts. Conventional dispersants such as those sold under the trade names Dolapix A-88 or Dolapix CE-64 can be used. The dispersant is, therefore, a non-reactive surfactant type material.

Without wishing to be bound by the theory, the inventors provide that the precipitating compound causes the starting metal salts, such as a nitrate, to undergo a reaction, for example, to the corresponding hydroxide and carbonate salts. These salts can be those that are deposited on the surface of the ceramic particles and become the oxide during the calcination.

Therefore, this process allows the formation of a coating such as a hydroxide or carbonate coating on the ceramic particles. Since the metal salts are preferably soluble in water, it is believed that there will be no free metal salt particles formed in the suspension. In addition, it is also preferred if the precipitator compound is soluble in water. Therefore, there should be no particles formed from metal salt or precipitant compound.

In one embodiment, a metal sol is used as boehmite [AlO (OH)] in the precipitation methods, or it is generated during the precipitation process. Consequently, particles such as silicon carbide particles are mixed with the metal sol precursor. The precipitated compound is then added with titration, ideally until the pH of the suspension is between 9 and 11.

In a more preferred embodiment, a mixture of Al (NO3) 3 + Y (NO3) 3 is used in the method of the invention. The molar ratio of these metal salts can be 5: 3, since it forms a eutectic and produces yttrium and aluminum garnet (YAG) after calcination and sintering.

Once the coating has been produced, the particles can be filtered from the remainder of the suspension and the particles dried, preferably spray-dried using conventional lyophilization processes. The coated particles formed in this stage of the process tend to agglomerate and may have particle sizes of 10 micrometers or more, such as 15 micrometers or more, such as 20 to 50 micrometers.

When a hydrogen carbonate precipitator is used, it is preferred that the particles are spray dried, although conventional oven drying is also possible. However, when a urea precipitator is used, the coated particles are preferably baked before further processing (such as calcination, sintering, sieving, etc.).

The process of coprecipitation with AHC can be spray dried directly after the unfiltered titration process. However, when a urea precipitator is used, it is preferred if the filtration occurs and the filtrate is combined with fresh distilled water (and with the optional addition of PVA and PEG). The solids content can be increased to 20 to 40% by weight to reduce the cost of drying.

5

10

fifteen

twenty

25

30

35

40

Four. Five

fifty

55

60

65

The coated particles are calcined. Calcination can occur at conventional temperatures and using conventional techniques. A temperature of 400 to 800 ° C, such as 500-600 ° C is preferred. A temperature of 800 to 1200 ° C, such as 900 to 1000 ° C, is also possible, but is less preferred. The process is carried out in the presence of air to ensure the oxidation of the hydroxide to an oxide coating.

After calcination, the particles can be sintered. The sintering of SiC particles coated with oxide preferably takes place in an argon atmospheric furnace with a temperature of up to 2000 ° C, such as up to 1750 ° C. Ideally, the sintering should be carried out at a temperature of 1000 ° C to 2000 ° C, such as 1300 to 1800 ° C.

The sizes of the particles are again around 20 to 100 micrometers at the end of the calcination and sintering process.

Because the amount of salt or metal salts added to form the coating is relatively high, this means that the thickness of the coating and therefore also the oxide coating is thicker than the coating that could be given if, for example, introduces a coating simply as a sintering aid. The presence of a thick coating means that the oxide coating is able to prevent the degradation of the ceramic part during the thermal spraying. Instead of decomposing or sublimating, the ceramic partroula can be thermally sprayed successfully.

Therefore, it is provided that the ceramic particles coated with metal oxide of the invention comprise at least 5% by weight, such as at least 10% by weight, preferably at least 20% by weight of the oxide coating. The oxide coating ideally forms from 11 to 40% by weight of the ceramic particles coated in their entirety or from 10 to 30% by weight. The% by weight coating of the SiC particles can be calculated quantitatively according to the XRD pattern using the Rietveld method.

The thickness of the particulate oxide coating in the ceramic particles may preferably be in the range of 50 to 200 nm. Naturally, it is generally observed that the thicker coatings are present in larger particles.

The inventors provide that the coating will form a complete coating around the ceramic partroula. Any break in the coating could offer the possibility of its decomposition. Our coating, therefore, can be considered continuous. That said, even if there is a possibility that the oxide coating will break, perhaps during the manufacturing process or the thermal spraying process, the desired result can still be achieved. During the thermal spraying process, the oxide coating melts. Therefore, the oxide coating can cover any break in the coating while bonding the layers of the spray coating material.

It will be appreciated that prior to spray drying or prior to calcination and sintering, some binders (additives) could be added as is known in the art to ensure successful drying processes. Polyvinyl alcohol (PVA) can be added to aid in agglomeration to create a round-shaped powder. PEG can be added to increase the flowability of the suspension, which prevents clogging of the spray-drying nozzle and allows easy transfer of the spray-dried powder, etc.

The process of the invention results in the formation of agglomerated and sintered ceramic powders containing, among others, yttrium garnet and aluminum coated on each ceramic partroula.

After sieving (and prior to thermal spraying), preferably 20-45 micron-sized powder may be used as a raw material for high-frequency pulse detonation or high-speed oxy-fuel thermal spraying techniques. Larger powders that have a size of 45-90 micrometers can be used for the spraying of atmospheric plasma.

Thermal spraying

The particles formed after calcination can be thermally sprayed onto a substrate. Various thermal spraying techniques could be used, such as those based on combustion (eg, flame spraying or HVOF), detonation (detonation gun or high frequency detonation gun) or electric / plasma spraying (plasma spraying). atmospheric, wire arc spraying, low pressure plasma spraying or vacuum plasma spraying). Preferred spray techniques include a high frequency detonation gun, an HVOF technique or an atmospheric plasma spray. As noted above, these techniques are well known and a complete summary of them is not required in this document.

The use of a detonation gun is preferred and is explained in detail in US6745951. A detonation pistol for thermal spraying is formed by a combustion chamber and a canon, with inputs for

5

10

fifteen

twenty

25

30

35

40

Four. Five

fifty

55

60

65

fuel and for oxidant. It is also provided with one or more bumps to detonate the fuel-oxidant mixture and one or more injectors for the introduction of the product into the canon.

High-speed oxy-fuel spraying involves a mixture of gaseous or liquid fuel and oxygen that is fed into a combustion chamber, where they ignite and burn continuously. The resulting hot gas at a pressure close to 1 MPa emanates through a converging-diverging nozzle and travels through a straight section. The speed of the jet at the exit of the canon (> 1000 m / s) exceeds the speed of sound. The feed material is injected into the gas stream, which accelerates the powder up to 800 m / s. The stream of hot gas and dust is directed towards the surface to be coated. The powder partially melts in the stream and deposits on the substrate.

In plasma spraying processes, the material to be deposited is introduced into the plasma jet, which emanates from a plasma torch. In the jet, where the temperature is of the order of 10,000 K, the material melts and propels towards a substrate. There, the molten droplets flatten, solidify quickly and form a deposit.

The substrate on which the coated particles are thermally sprayed is not limited and, therefore, can be any substrate of interest to the skilled artisan. The present inventors have a particular interest in spraying the particles on the mechanical working parts of large industrial equipment, such as wind turbines.

Therefore, the substrate is preferably a metal substrate such as steel and its alloy, aluminum and its alloy and other metal or a polymer substrate.

The thickness of the coating on the substrate may vary depending on the thermal spray parameters. Thicknesses of 10 to 500 micrometers (microns) are possible, preferably 100 to 200 micrometers (microns).

The coatings formed on the substrate have excellent properties of resistance to wear and corrosion. In general, the coatings are rough (Ra = 4.2 micrometers (microns)) as pulverized. To improve performance in wear applications, it may be necessary to polish the sprayed surface until the surface is very smooth (Ra = 0.1 micrometers)). In order to maximize the efficiency of the thermal spraying process, it may be necessary to prepare the substrate surface for coating. The surface of the substrate must be clean. It can also be cleaned with grain or similar to create a rough surface that helps the adhesion of the coating during thermal spraying.

The invention will now be described in more detail with reference to the following non-limiting examples and figures.

Figure 1 shows a particle of the invention with silicon carbide coated with metal oxide with 30% by weight of YAG composition.

Figure 2 shows the XRD spectra of powders after the precalcining process at 500 ° C and the sintering process at 1750 ° C with respect to the content of the YAG phase of 30% by weight.

Figure 3 shows the scanning electron micrograph of the cross section of the coating of the powdered powders of the invention to show the nature of the coating. Keep in mind that the top layer of these electronic micrographs is simply an epoxy layer added to allow the generation of images.

Figure 4 is an enlarged view of the powdered powders of the invention to show the nature of the coating.

Figure 5 shows sintered and agglomerated SiC powders suitable for HFPD (24-45 micrometers). Example 1 - (Silicon carbide coated with oxide from metal salt)

This example is based on the following alleged reactions with hydrogen carbonate and ammonium (AHC):

1. Initial hydrolysis of AHC in distilled water:

NH4HCO3 + H2O ^ NH4OH + H2CO3 NH4OH ^ NH4 + + OH- H2CO3 ^ H + + HCO3-

HCO3- ^ H + + CO32

2. Reaction of aluminum nitrate

Al (NO3) 3 ■ 9H2O + 3NH4HCO3 = AlOOH + 3 (NH4) NO3 + 3CO2 + 10H2O (boehmite or aluminum hydroxide)

5

10

fifteen

twenty

25

30

35

If the suspension is aged several hours at room temperature, the hydroxide could react and form ammonic dawsonite

To (NOa) to ■ 9H2O + 4NH4HCO3 = NH4Al (OH) 2CO3 + 3 (NH) 4) NO3 + 3CO2 + IOH2O (ammonic dawsonite)

AlOOH + NH4HCO3 = NH4Al (OH) 2CO3

3. Reaction of yttrium nitrate

2Y (NO3) 3 ■ 6H2O + 6NH4HCO3 = Y2 (CO3) 3 ■ 6H2O + 6NH4NO3 + 3CO2 + 9H2O (normal carbonate hydrate) or

Y (NO3) 3 ■ 6H2O + 3NH4HCO3 = Y (OH) CO3 + 3 (NH4) NO3 + 2CO2 + 7H2O (basic carbonate) The following flow chart explains the process:

image 1

An agglomerated powder of silicon carbide coated with metal oxide was prepared for the thermal spray feed material containing 30% by weight of YAG by a co-precipitation method of a metal salt precursor on a silicon carbide particle. . The silicon carbide particles used have an average granulometry of 0.6 micrometers (microns). The coated metal oxide was confirmed by the micrograph in Fig. 1 and Fig. 2 showing the YAG phase of coated silicon carbide and an X-ray crystallogram of the coated silicon carbide powder, respectively.

The YAG phases that cover the surface of silicon carbide are a eutectic phase of A ^ O3 and Y2O3 that result from the calcination of a co-precipitation of metal hydroxide coated with a mixed solution of Al (NO3) 3 ■ 9H2O and Y ( NO3) 3 ■ 6H2O according to the molar ratio Y: Al = 5: 3. The designed stoichiometric composition gives rise to 30% by weight of YAG phases.

Coprecipitation of the YAG phase was initiated by dispersing 100 grams of silicon carbide particles in distilled water. About 0.4% by weight of dispersant is added to stabilize the silicon carbide suspension. The dispersant was a Dolapix A-88 dispersant from Zschimmer & Schwarz GmbH & Co KG. Magnetic stirring was used to homogenize the suspension. 721.96 ml of 6.4 M ammonium hydrogencarbonate precipitating agent was added to the SiC suspension. The suspension was heated to 50 ° C before the mixed metal salt (formed by 721.96 ml of Al (NO3) 3 ■ 9H2O 0.5 M and 721.96 ml of Y (NO3) 3 ■ 6H2O 0.3 M) was titrated in the suspension mixture. The suspension that now contains particles of silicon carbide coated with hydroxide / metal carbonate is then filtered and washed with distilled water.

5

10

fifteen

twenty

25

30

35

40

Four. Five

fifty

55

60

65

The treated powder is dried in the oven and preheated at 500 ° C for 2 hours in an air oven. The sintering was used for the agglomerated powders in a forging or argon environment at 1750 ° C for 2 h.

In a second experiment, the treated SiC powders were spray dried by introducing distilled water to produce a solid content of 20-40% by weight. Polyvinyl alcohol and polyethylene glycol are added to aid in the spray drying process. The spray-dried powders are pre-calcined at 500 ° C for 2 hours in an air oven before sintering at 1750 ° C for 2 h in a forging or argon environment.

In both processes, the agglomerated and sintered powders must be sieved to ensure an exact distribution of the powder size according to the type of thermal spraying methods. After spray drying we can obtain the desired particle size. However, during sintering, shrinkage and agglomeration occurs causing the size of the particles to deviate. In this way sieving is used to classify the agglomerated powders. Figure 5 shows the raw material for HFPD.

Example 2 - Substrate coating

The powder of example 1 (oven-dried) with powders of 20-45 pm was sprayed onto a carbon steel substrate with a high frequency pulse detonation gun according to the following parameters

Gas flow: 48 SLPM of propylene plus 170 SLPM of oxygen (SLPM = standard liters per minute)

Frequency: 60 Hertz.

Distance from the torch to the substrate: 40 mm.

Powders that are fed with Thermico CPF-2 employ 20 SLPM nitrogen carrier gas and feed disk at 10 rpm of rotation.

Number of sweeps of the torch on the substrate: 4 x 6 seconds.

At the end of the deposition, the coating samples were characterized with X-ray crystallography. Fig. 2 shows the crystallogram of the deposition of powder containing SiC + 30% by weight of YAG. The X-ray crystallogram of the powder before deposition and coating are practically identical. This means that no decomposition has occurred in the thermal spraying process. In Fig. 2, the maximum positions of the two compounds are marked.

An electronic scanning micrograph is made of the cross section of the coating, which results in the structure as shown in Fig. 3. The typical structure in which the YAG phases (shown as the brighter area) surround silicon carbide ( shown as a darker area) as expected from the coating deposition produced with these powdered raw materials.

The agglomerated particles have a size of about 45 micrometers (microns). When they travel in the flame during thermal spraying, they deform as the oxide melts and flatten upon contact with the substrate. As more particles come into contact with the substrate, a coating layer begins to form (with each layer only 5-10 micrometers (microns) thick). Therefore, a coating of more than 100 micrometers (microns) can easily be achieved on a substrate as shown in Fig. 3.

Careful study of the coating increase shows that it is composed of small SiC particles joined by the oxide phase (Figure 4).

Example 3

The process of example 1 was repeated, but this time with 3 micrometer silicon carbide particles (microns).

Example 4 - (Silicon carbide coated with oxide of a metal salt precursor with urea precipitator)

The encapsulated oxide will protect the SiC particles from the direct interaction with the plasma, thus inhibiting the decomposition.

Ingredients:

to. Partroulas of a-SiC with d50 = 0.6 pm supplied by Washington Mills AS, Orkanger, Norway

b. Al (NOa) to ■ 9H2O (Merck KGaA, Germany)

c. Y2O3 from H.C. Starck Grade C (dissolved in HNO3 solution to produce 2Y (NO3) 3 ■ 3H2O after the reaction of Y2O3 + 6HNO3 + 9H2O ^ 2Y (NO3) 3 ■ 6H2O.

d. Weak base precipitator: Urea

and. Dispersant: Dolapix A88 (Zschimmer & Schwarz GmbH & Co. KG., Germany)

5

10

fifteen

twenty

25

30

35

The following flow diagram explains the process:

image2

Procedures:

The total solid charge after the coprecipitation process is approximately 5% by weight of the molar ratio of urea / (Al3 + + Y3 +) = 7.5

100 g of a-SiC particles were dispersed in 721.96 ml of 6 M urea and stirred at 600 rpm for 30 minutes. Dolapix A88 at 0.4% by weight was added to stabilize the SiC suspension containing stirred urea for 15 minutes. The mixture was heated to 90 ° C. Separately, a salt metal precursor YAG solution (ratio of Al3 +: Y3 + = 5: 3) was prepared by combining 721.96 ml of 0.5 M Al (NO3) 3 ■ 9H2O and 721.96 ml of 0.3 M of Y (NO3) 3 ■ 6H2O. This will give a yttrium and aluminum garnet content (Y3AbO12) of 30% by weight when sintering. The metal salt precursor solution was reverse-titrated in the SiC suspension at a flow rate of 3 ml / min. The pH of the suspension was maintained at 9 or more with the addition of NH 4 OH.

Once the titration process was finished, the suspension was aged for 1 hour (aging = agitation at 600 rpm with the temperature maintained at 90 ° C for one hour). The aged suspension is filtered to remove the excess NO32- ion. The SiC supernatant is then dried in the oven at 100 ° C for 24 hours.

The dry SiC is previously calcined at 500 ° C for 2 hours to eliminate the hydroxide and carbonate species that form the coating at this stage of the process. An additional sintering was performed at 1750 ° C in an argon atmospheric furnace to crystallize and sinter the agglomerated SiC powders.

The agglomerated and sintered powders are classified with a sieving machine to produce powders of 25-45 pm and 45-90 pm. The agglomerated powders of 25-45 pM are mainly used for the high frequency pulse detonation gun (HFPD), while the modified SiC powders of 45-90 pM are mainly used for the atmospheric plasma spraying system (APS) ).

The following theoretical reactions can take place:

Reactions with urea:

1. Initial hydrolysis of urea in distilled water

CO (NH2) 2 ^ NH4 + + OCN- (initial hydrolysis)

OCN- + 2H + + H2O ^ CO2 + NH4 + (in acid medium)

OCN- + OH- + H2O ^ NH3 + CO32 "(in neutral or basic solution)

5

10

fifteen

twenty

25

30

35

40

Four. Five

fifty

55

60

65

2. Reaction of aluminum nitrate

2Al (NOa) to ■ 9H2O + 3CO (NH2) 2 ^ 2AIOOH + 6 (NH4) NO3 + 3CO2 + IIH2O (boehmite or aluminum hydroxide)

or

Al (NO3) 3 ■ 9H2O + 2CO (NH2) 2 ^ NH4Al (OH) 2CO3 + 3 (NH4) NO3 + CO2 + 4H2O (ammonium dawsonite)

3. Reaction of yttrium nitrate

2Y (NO3) 3 ■ 6H2O + 3CO (NH2) 2 ^ Y2 (CO3) 3 ■ 6H2O + 6 (NH4) NO3 + CO2 + 5H2O (normal carbonate)

Y3 + + H2O = (Y (OH)) 2 + + H + (Y (OH)) 2+ + CO2 + 2H2O = Y (OH) CO3 ■ H2O + 2H + (basic carbonate)

Powders formed by oven drying or spray drying in Examples 1, 3 and 4 can be thermally sprayed by HFPD, HVOF or APS.

Example 5 Thermal spraying process

Spraying process of atmospheric plasma.

The powder of Example 1 (oven-dried) with powders of 45-90 μm was sprayed onto a carbon steel substrate with an atmospheric plasma spray gun F4-MB plasma gun with a 6 mm anode diameter installed in an A3000S plasma spraying system (Sulzer Metco, Wolhen, Switzerland) according to the following parameters:

Gas flow: 45 SLG of Argon plus 12 SLPM of Hydrogen (SLPM = standard liters per minute)

Current: 700 amps Voltage: 47 volts Injector diameter = 1.8 mm

Distance of the plasma torch from the substrate: 100 mm.

Rotation of digital powder feed: 20 rpm with carrier gas Ar at 2.8 SLPM Robot movement 0.2 m / s.

Number of sweeps of the torch on the substrate: 4 x 6 seconds.

At the end of the deposition, the coating samples were characterized with X-ray crystallography. Fig. 2 shows the crystallogram of the powder feed material containing 30% by weight of YAG and the SiC composite coating produced using HFPD and APS. The X-ray crystallogram of the SiC phase before deposition and in the coating are practically identical. This means that SiC decomposition has not occurred in the thermal spraying process.

When the agglomerated powders travel in the flame during thermal spraying, they deform as the oxide melts and flatten upon contact with the substrate. As more particles come into contact with the substrate, a coating layer begins to form (with each layer only 5-10 micrometers (microns) thick). Therefore, a coating of more than 100 micrometers (microns) on a substrate can be easily reached.

Careful study of the coating increase shows that it is composed of small SiC particles joined by the oxide phase.

Example 6 - (Silicon carbide coated with oxide from the spray drying process)

The precursor of the metal salt spray dried mixed with SiC particles will produce encapsulated SiC particles of the hydroxide or carbonate metal precursor. As this process creates agglomerated SiC powders with nanoprecipitate of metal precursor, the sintering process creates SiC powders coated with YAG.

Ingredients:

to. Particles a-SiC with d50 = 1 pm supplied by Saint Gobain Ceramic Materials AS Lillesand, Norway

b. Al (NO3) 3 ■ 9H2O (Merck KGaA, Germany)

c. Y2O3 of HC Starck Grade C (dissolved in HNO3 solution to produce Y (NO3) 3 ■ 6H2O after the reaction of Y2O3 + 6HNO3 + 9H2O ^ 2Y (NO3) 3 ■ 6H2O.

d. Acid weak precipitator: Cttric acid

and. Dispersant: Dolapix A88 (Zschimmer & Schwarz GmbH & Co. KG., Germany)

5

10

fifteen

twenty

25

30

The following flow diagram explains the process:

image3

Procedures:

The total solid charge after the co-precipitation process is approximately 10% by weight of the molar ratio of the acid / / (Al3 + + Y3 +) = 3

100 g of a-SiC particles were dispersed in 360.98 ml of 4.8 M duric acid and stirred at 600 rpm for 30 minutes. Dolapix A88 at 0.4% by weight was added to stabilize the suspension of SiC containing dicaric acid and stirred for 15 minutes. The suspension was heated to 80 ° C.

Separately, a salt solution of YAG salt precursor metal (ratio of Al3 +: Y3 + = 5: 3) was prepared by mixing 360.98 ml of Al (NO3) 3 ■ 9H2O 1 M and 360.98 ml of Y (NO3) ) 3 ■ 6H2O 0.6 M. This will provide a yttrium aluminum content (YsAkO ^) of 30% by weight upon sintering. The metal salt solution was poured stepwise into a suspension of SiC and stirred for 1 hour.

A spray dryer was heated until the inlet temperature reached 210 ° C and the outlet temperature was stable between 90 and 110 ° C. The SiC / metal salt solution mixture is spray dried, adjusting the feed of the suspension and the aspirator to produce spherical agglomerated SiC with the desired particle size between 25 and 90 micrometres (microns). These particles were calcined at 500 ° C for 2 hours in the atmospheric furnace to eliminate the hydroxide and carbonate species formed during spray drying. The additional sintering at 1750 ° C is carried out in an argon atmospheric furnace to crystallize and sinter the agglomerated SiC powders. The agglomerated and sintered powders are classified with a sieving machine to produce powders of 25-45 pm and 45-90 pm. The agglomerated powders of 25-45 pM are mainly used for the high frequency pulse detonation gun (HFPD), while the modified SiC powders of 45-90 pM are mainly used for the atmospheric plasma spraying system (APS) ).

Claims (13)

5 10 fifteen twenty 25 30 35 40 Four. Five fifty 1. A process for thermally spraying ceramic particles coated with metal oxide on a substrate comprising: (i) obtaining a plurality of particles coated with metal oxide of silicon carbide, silicon nitride, boron carbide or boron nitride in which the metal oxide coating forms at least 5% by weight of the weight of the coated particle ; Y (ii) thermally spraying the particles of step (I) onto a substrate; wherein the metal oxide is yttrium and aluminum garnet and in which the coating forms a continuous and complete coating around the ceramic particle. 2. A process according to claim 1, wherein said plurality of particles coated with metal oxide is obtained by calcining and sintering a plurality of particles coated with metal salt of silicon carbide, silicon nitride, boron carbide or boron nitride, such as particles coated with metal hydroxide and / or metal carbonate. 3. A process according to claim 2, wherein said plurality of particles coated with metal salt is obtained by combining a plurality of particles of silicon carbide, silicon nitride, boron carbide or boron nitride with at least one metal salt, such as two metal salts, in the presence of a weak acid or a weak base to form a metallic salt coating on said particles; and drying, as by spray drying, the resulting particles. 4. A process according to any preceding claim in which the particles are particles of silicon carbide. 5. A process according to claim 3 or 4, wherein the metal salt is a mixture of yttrium nitrate and aluminum. 6. A process according to claims 3 to 5, wherein there is from 1 to 30 times the molar ratio of weak base or weak acid to salt or metal salts, preferably from 1 to 3 molar equivalents when a weak acid is used and 6 to 8 molar equivalents when a weak base is used. 7. A process according to claims 3 to 6, wherein the salt or metal salts are titrated in a solution of the weak base or weak acid with particles or in which the base or the weak acid is titrated in a solution of the salt or metallic salts and the particles. 8. A process according to claims 3 to 7, wherein the salt or metal salts, the weak base or the weak acid and the particles are combined and spray dried to coat the particles with the metal salt coating. 9. A process according to any preceding claim wherein the thermal spraying is effected using a detonation gun. 10. A process according to claims 3 to 9, wherein the weak base is urea, ammonia or a hydrogen carbonate, especially ammonium hydrogencarbonate and the weak acid is citric acid. 11. A process according to any preceding claim wherein the substrate is metal. 12. A process according to any preceding claim, wherein the metal oxide coating forms at least 10% by weight of the weight of the coated particle before the thermal spraying process. 13. A process according to claims 2 to 12, wherein the particles are coated with a hydroxide and / or carbonate prior to calcination.